The present invention relates to a fuel cell system, comprising at least one fuel cell, wherein the fuel cell comprises an anode chamber and a cathode chamber separated from the anode chamber, and wherein the fuel cell system also comprises a cathode gas source, a gas supply line, which is connected to the cathode gas source, and an exhaust air line, which is connected to the cathode chamber. The preset invention further relates to a method for operating a fuel cell by means of the fuel cell system.
Fuel cells are separated for safety reasons from the high-voltage (HV) network or the on-board electrical system by means of contactors, in particular primary contactors, of the fuel cell system. The contactors are closed only during operation, for example during operation of a vehicle operated with the fuel cell, and the fuel cell is connected to the HV on-board electrical system. When the vehicle is shut down, i.e. when the vehicle is switched off, the primary contactors are again opened and the fuel cell is again separated from the HV on-board electrical system of the vehicle until the next start-up process.
During a start-up procedure of the vehicle, the fuel cell is separated from the HV on-board electrical system in a known manner. This is due to the fact that, according to the prior art, high differences in voltage of up to several hundred volts can occur between the fuel cell and the on-board electrical system at the end of the start-up process. These differences in voltage occur because the fuel cell or, respectively, a fuel cell stack is heated up in the first step of the process when starting the vehicle under freezing conditions. This is done in order to prevent the gas channels from freezing during normal operation. After the fuel cell or fuel cell stack has therefore achieved a sufficient voltage of, for example, 400 V, a precharging resistor comprising its own contactor is generally used in order to prevent damage to the on-board electrical system. In so doing, a small current flows through the relatively high-impedance resistor.
The precharging resistor comprising its own contactor is in a known manner either integrated in the DC/DC converter downstream of the fuel cell or, respectively, fuel cell stack or is constructed from discrete components. Because the required components for integrating a precharging resistor comprising its own contactor into the DC/DC converter or the individual components are used in the HV range, said components contribute decisively to the system costs of the fuel cell system. In addition, the components for the precharging resistor comprising its own contactor take up a significant amount of installation space.
A precharging resistor comprising its own contactor in a fuel cell system is known, for example, from the Japanese patent application JP2005295697A. The function of the precharging resistor is to match the fuel cell voltage with an on-board electrical system voltage. If the voltage between the on-board electrical system and the fuel cell or, respectively, the fuel cell stack is matched via the precharging resistor, the primary contactors are closed and the current flows through the main path which connects the fuel cell to the on-board electrical system. In the process, the contactor of the precharging resistor is opened and the start-up process of the fuel cell is completed after this step.
The German patent application DE 101 27 892 A1 discloses a start control device for a vehicle comprising a fuel cell. During the start-up process of the fuel cell, a circuit comprising a current limiter is used, said circuit preventing a direct connection of the fuel cell to a load network comprising an energy storage unit. This is performed inter alia by the current limiting device until a difference between the output voltage of the fuel cell and a voltage of the energy storage unit achieve a predetermined voltage difference. As a result, an inflow of high current from the fuel cell to the energy storage unit is prevented according to said German patent application DE 101 27 892 A1.
The German patent application DE 10 2009 023 882 A1 discloses a system and a method for improving the start-up reliability of a fuel cell system. Provision is thereby made for a high-frequency resistance measurement of the fuel stack to be provided, wherein one or a plurality of correction measures can be taken on the basis of the measurement results in order to prevent the membranes of the fuel cell stack from drying out. One of these correction measures can entail limiting an airflow to the cathode side of the fuel cell stack. This has the result of reducing the drying effect of the cathode air on the membranes.